THERE WAS NO IMMEDIATE OR IMMINENT THREAT TO SLAVERY IN 1861... 

If the South would NOT have seceded, Lincoln would NOT have invaded the South to free slaves. That was never his motive.

Slavery was protected by the US Constitution. Several states that remained in the Union continued slavery and even the Lincoln administration was using slave labor to complete construction of the Capitol building. 

West Virginia was admitted into the Union in June of 1863 as a slave state 6 months after the Emancipation Proclamation went into effect. 

Alexander Stephens VP of the Confederate States said that slavery was "safer in the Union than out of it." 

The fugitive-slave clause which Lincoln supported, protected slavery even in free states: If a slave escaped to a free state, his status remained that of a slave. In 1850, the Fugitive Slave Law even required the capture and return of runaway slaves by citizens of the North.

Logic would suggest that there must be another reason for invading the South....